Abstract: :
Purpose: The electroretinographic (ERG) b–wave intensity–response(I–R) function has been used extensively to analyze retinalfunction in patients with retinal disease. It provides moreinsights about retinal pathophysiology than does the "standard"scotopic ERG, in which only two stimulus intensities are used.The traditional approach for recording I–R functions involvesthe use of 10 or more stimulus intensities. This requires aconsiderable increase in testing time in comparison with thatrequired for the standard ERG. The aim of this study was todetermine if reliable I–R function estimates could beobtained using a small number of stimulus intensities.Methods: Twenty control subjects with normal ERG I–R functionparameters, and twenty patients, in whom at least one of thefunction parameters was abnormal, were included in this study.ERGs were recorded following ISCEV recommendations after darkadaptation. The standard flash intensity of 1.64 cd.s.m–2was attenuated from 4.0 to 0.0 log units in 0.2 log unit steps.I–R functions were calculated using either an extendedprotocol (all suprathreshold values) or an abbreviated protocol(the subset of values obtained with 3.2, 2.4, 1.6, 0.8 log unitsattenuation). The "second limb" was not used in the fit in eithercase. The usual Fulton–Rushton model (V = (Vmax . In )/(In + kn)) was used to fit I–R functions, where Vmax isthe plateau, k the semi–saturation constant, and n theslope.Results: The four data points used in the abbreviated protocolwere sufficient to obtain a fit of the I–R function inall cases. The mean goodness of fit, assessed with the correlationcoefficient, was 0.99 in both cases. Differences between thetwo methods were estimated using the Wilcoxon test. No significantdifferences were seen between the parameters obtained with theextended vs. the abbreviated protocol (mean Vmax: 341.54 vs.345.03 µV, p = 0.677) (mean k: – 2.34 vs. –2.30 log cd.s.m–2, p = 0.12) (mean n: 0.99 vs. 1.00, p= 0.294). The results were similar when the control group andthe patient group were analyzed separately.Conclusion: While the use of a large number of data points,from b–wave threshold to plateau, remains the "gold standard"to obtain accurate I–R function parameters, a very reasonableestimate may be achieved using a limited number of well–chosendata points. With only a minimal increase in recording timein comparison with the standard ERG protocol, one can thus extractconsiderably more information about a patient's retinal function.